BACKGROUND: In industrialized countries a considerable and increasing proportion of strokes occur at younger ages. Stroke at young age causes marked disability at worst and thus long-standing socioeconomic consequences and exposes survivors for 4-fold risk of premature death compared with background population. Up to 50% of young patients with ischemic stroke remain without definitive etiology for their disease despite extensive modern diagnostic work-up (i.e. cryptogenic stroke). The group of cryptogenic strokes includes those with patent foramen ovale (PFO) or other abnormalities in the atrial septum in the heart as the only or concomitant finding. Population prevalence of PFO is high, 25%, and the mechanisms how PFO would be associated causally with ischemic stroke remain to be clarified. Moreover, there are only scarce data on clinical outcome, long-term risk of new vascular events, and prevention of such events in these patients.